What is Constitutional Avoidance?

Legal Definition
In United States constitutional law, the doctrine of constitutional avoidance dictates that a federal court should refuse to rule on a constitutional issue if the case can be resolved on a nonconstitutional basis. When a federal court is faced with a choice of ruling on a statutory, regulatory or constitutional basis, the Supreme Court has instructed the lower court to decide the federal constitutional issue only as a last resort: "The Court will not pass upon a constitutional question although properly presented by the record, if there is also present some other ground upon which the case may be disposed of." Ashwander v. Tennessee Valley Auth., 297 U.S. 288, 347 (1936) (Brandeis, J., concurring).

The avoidance doctrine flows from the canon of judicial self-restraint, and is intertwined with the debate over the proper scope of federal judicial review and the allocation of power among the three branches of the federal government and the states. It is also premised on the "delicacy" and "finality" of judicial review of legislation for constitutionality, concerns regarding the credibility of the federal courts, and the "paramount importance of constitutional adjudication in our system." These elements demonstrate a significant overlap between the avoidance doctrine and other jurisdictional or justiciability barriers. The avoidance doctrine reflects such other justiciability doctrines as standing and ripeness, and permeates jurisdictional doctrines such as Pullman abstention and the adequate and independent state ground doctrine.
-- Wikipedia
Legal Definition
Constitutional Avoidance is the principal that, if possible, the Supreme Court should avoid ruling on constitutional issues, and resolve the cases before them on other (usually statutory) grounds. In practice, what this often means is that if the Supreme Court is faced with two possible interpretations of a statute, one of which is plainly constitutional, and the other of which is of questionable constitutionality, the court will interpret the statute as having the plainly constitutional meaning, to avoid the hard constitutional questions that would come with the other interpretation.